Is the lagoon fed by a spring, or is it just 'run-off' which accumulates? In other words, does it dry up in summer or is the level constant, with a little stream leaving the lagoon?
If run-off only, I would fill it with old bales of hay (hay after htree years is pretty much cellulose dust anyway) to soak up the liquid, and allow it to do a bit of aerobic composting. At the end of the following summer, dump the lot into a muck spreader, and fertilise an area you want to make hay on next summer. Alternatively, just dump as many old bales of hay as it takes to soak up most of the slurry. Then leave it to overwinter, and plant squash in spring. After a couple of seasons - if it remains dryish, you'll have some excellent compost, and you could turn your lagoon into a first class vegetable patch - but make sure you control the run-off with ditch/drain and avoid it turning into a lagoon again.
If spring fed, I would try to clean out the lagoon to have a free source of clean water. Get hold of some 50mm diameter tube, and empty the lagoon into the field below, and CAREFULLY scrape out the slurry/foul mud on the bottom of the lagoon. You may need to do this a few times.
